The present invention concerns a device for reducing the jitter due to pointer adjustments in a digital telecommunication network.
The invention applies in particular to Synchronous Digital Hierarchy (SDH) networks. Clocks synchronized by one, or more, atomic clocks are used at different network nodes in order that they be highly stable, for instance of the order of 10.sup.-11. However this stability is not perfect and differences in phase and in frequency occur between the clock signals of the various nodes which must be allowed for without losing useful data. This purpose is assigned to the pointer adjustments. When the phase difference between an incoming signal at the node and the local clock signal exceeds a given threshold, a positive or negative pointer adjustment is carried out to relock the signals in phase.
To understand the phenomenon of a pointer adjustment, it must be borne in mind that the signal moves in frames. A frame is a set of binary elements, or bits, of a specified number, which is conveniently represented as a block of octets comprising a given number of rows or lines, each line comprising in turn a specific number of octets, i.e. 8-bit bytes. Illustratively in the case of the STM-1 (Synchronous Transport Module--Level 1) signal defined by the CCITT, a frame contains 19,440 bits arranged on 9 lines each of 270 octets. Not all the octets of a frame serve to move useful information, a number of them serve to monitor and control the transmission. As regards the illustrative STM-1, each line starts with 9 monitoring and controlling octets, called SOH (Section Overhead) followed by an octet called POH (Path Overhead), then by 260 octets which can be divided into 20 zones each comprising 13 octets. Each of these 20 zones comprises 12 useful-information octets and one type W, X, Y or Z octet, using the CCITT terminology. A W octet comprises 8 useful information bits, an X or Y octet includes no useful information, and a Z octet comprises 6 or 7 useful data bits. In what follows below, and in spite of the above discussion, the 261 octets following the 9 SOH octets are useful-information octets. Thereby the discussion shall be simpler without reducing generality.
In the light of the above assumption, a frame therefore carries 9.times.261=2,349 octets of useful information and 9.times.9=81 SOH octets for monitoring and control. Among the latter, 3 octets however are preserved to move additional useful information if called for by the phenomenon of pointer adjustments.
As already noted, a pointer adjustment may be positive or negative in order to properly relock the phase regardless of the phase-shift sign involved in the pointer adjustment.
A positive pointer adjustment of a frame means that this frame carries a number of useful data less than the number of useful data in a normal frame unaffected by an pointer adjustment. As regards the illustrative STM-1 signal, any positive or negative pointer adjustment evinces an amplitude set at 3 octets, that is 24 bits. For a frame subject to a positive pointer adjustment, there only moves 2,346 useful data octets because a frame without pointer adjustment moves 2,349. The place preserved in the frame for the three missing octets of useful information is taken up by 3 dummy octets.
A negative pointer adjustment affecting this frame moves a number of useful data octets which is larger than the number of useful data octets of a normal frame. As regards the STM-1 signal, a frame subjected to a negative pointer adjustment therefore moves 2,352 useful data octets because a frame lacking a pointer adjustment moves 2,349. As a priori, a frame is provided with only 2,349 octets for moving useful data, in this case the useful data must be made to spill over the monitoring and control data by using the reserved 3 monitoring and control octets already mentioned.
To process and regenerate useful information moved by a signal transmitted through the network, said signal is made to generate in known manner a data signal and a clock signal Again in known manner, the transitions denoting the monitoring and control bits are eliminated from the clock signal in order to have available a modified clock signal of which the transitions evince only useful information bits. Such a modified clock signal when, for instance, applied to the write input of a memory comprising an input receiving the data signal, allows storing into this memory only the useful information bits while ignoring the monitoring and control bits Hereafter, this modified clock signal shall be called the write clock or write clock signal.
Accordingly, a priori, the write clock is in the form of bursts, each including a plurality of clock pulses separated by stages without any clock pulse. The bursts of clock pulses correspond to the useful information bits of the transmitted signal whereas the intervals correspond to the monitoring and control bits of this transmitted signal In principle, each frame is transmitted line by line. In the case of the illustrative STM-1 signal and in the light of the simplified discussion herein, as a rule, each line comprises initially 9 SOH monitoring and control octets which are followed by 261 octets of useful information. Accordingly the clock write signal ordinarily comprises intervals with a duration corresponding to 9 octets, i.e. 72 bits which separate bursts comprising as many clock pulses as there are useful information bits per line, in this case therefore 2,088 clock pulses.
This holds for all lines of frames not subjected to a pointer adjustment. However as soon as a pointer adjustment materializes for a frame, this frame includes a line in which the number of useful information bits no longer is 2,088. Consequently, that part of the write clock signal corresponding to this line either will evince a lengthening of the interval followed by a shortening of the next burst, or a shortening of the interval followed by a lengthening of the next burst.
As regards a positive pointer adjustment in a frame of an STM-1 signal, the interval shall be 96 bits instead of 72 because the first 24 bits ordinarily assigned to the useful information of the line in this case are dummy bits which need not be processed and stored. The following burst then comprises 2,064 clock pulses instead of 2,088.
As regards a negative pointer adjustment in a frame of an STM-1 signal, the interval is 48 bits instead of 72 because the last 24 bits ordinarily assigned to monitoring and control in this case are useful information bits which must be processed and stored. Then the following burst comprises 2,112 clock pulses instead of 2,088.